Soil like in earth, ground and geo is a building material like steel, concrete and wood. The difference is that the physical properties of soil are unknown at the start of design and construction, unlike most other building materials. This is because soils change with almost every step we take across some building sites.
It’s not a very glamourous material either, but everything in the built environment is supported on soil, occasionally bedrock – so, ignore the soil at your peril.
All structures in the built environment consist of:
- the upper part that you see,
- that is supported by a foundation that you don’t see,
- that is in turn supported by soil that is even deeper out of sight.
Studies in England found that most foundation failures occur because the physical properties of the soil are not determined properly.
A geotechnical engineers’s job is to identify the different layers of soil beneath a site. As part of the process they test the physical properties of the different soils in the field – insitu testing – and in the laboratory. They use these properties to design the soil component of a structure.
Geotechnical engineering, or soils engineering as it was known in the beginning, is one of several civil engineering specialties. (Figs 1, 2)
For example, Structural engineers design the steel, concrete and timber to support the upper part of the structure that you see. Foundation engineers design the footings and piles that you don’t see to support the structure above that you do see. Geotechnical engineers find a layer of soil at the construction site that is strong enough to support the foundations that support the structure. Construction engineers build it. If anything goes wrong, like a failure or a slip and fall accident, Forensic engineers determine the cause.
Geotechnical engineering has been practiced for 1,000s of years – all the way back to the Great Pyramid of Giza, 4,600 years ago – but wasn’t known as such. Back then it would have been a civil engineer who knew how to use soil in design and construction.
It came into it’s own in the early 1900s when an engineer by the name of Karl Terzaghi studied soil and it’s physical properties and developed analytical procedures for using it with steel and concrete in design and construction. Terzaghi is considered the Father of Soil Mechanics.
The soils beneath a site in Canada change as we walk across the site because the glaciers deposited different types of soils in different places 10,000 to 15,000 years ago. (Ref. 3) Farther south the soils result from the weathering and breakdown of the bedrock formed 1,000,000s of years ago. If the bedrock varies from place to place the resulting soils – residual soils – will vary too.
I’ve seen this in my engineering work with glacial soils in Canada and the U.K. and residual soils in the Bahamas and Australia.
Geotechnical engineers are interested in physical properties like the size and gradation of the soils – whether clay, silt, sand or gravel, or some combination – and the strength and compressibility of the soils.
They’re particularly interested in the bearing capacity and settlement characteristics of soils supporting foundations, the drainage properties of soils, and a soil’s susceptibility to slope failure, like along our highways.
(Bearing capacity is a soil’s ability to support the weight of a foundation. Soil settles or compresses when it’s doing this)
Analytical procedures have been developed by engineers like Terzaghi to use the physical properties of soil in design and construction in much the same way that the physical properties of steel, concrete and wood are used.
Understandably, considering the recent flooding, and the mudslides and landslides in British Columbia, geotechnical engineers are even more interested in preventing or avoiding these failures in soil when drenched with rain.
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Soil is everywhere beneath our feet and geotechnical or soils engineers – dirt doctors to some of our fellow civil engineers when they take a poke at us – are interested in everything to do with soil as a building material.
After years of engineering experience, I know it’s a simple building material once you pay attention and don’t ignore it. Google “What is geotechnical engineering, Wikipedia” and you will get a wealth of information, some quite technical. There’s a listing of some of this information in the Appendix. But really, it’s fairly simple.
Geotechnical engineers have well developed investigative and testing procedures to learn what’s beneath your feet on your building site and how to use it in design and construction.
If there’s an Achilles’ heel, observation and experience are important in the practice of geotechnical engineering. This is because geotechnical engineering relies on the semi-empirical science of soil mechanics.
Well experienced geotechnical engineers have been on site often and got their hands dirty and mud on their boots. Those who haven’t are in peril like the people who ignore soil during design and construction.
References
- What is civil engineering? Posted October 15, 2021
- What is forensic engineering? Posted September 28, 2021
- Sandford, R. W., The Columbia Icefield, Attitude Publishing, Banff, Alberta, Canada 1993
Appendix
A. A list of civil engineering specialties might look like the following to those of us who live in or near a city, town or village:
- Structural design engineering
- Foundation
- Geotechnical
- Construction engineering
- Highway engineering
- Environmental (formerly sanitary, water supply and storm water collection
- Forensic engineering
B. Types of structures in the built environment that a geotechnical engineer in which a geotechnical engineer might be involved:
- Foundations
- Lateral earth support structures
- Earthworks
- Geosynthetics
C. Some important properties of soils used by geotechnical engineers …
- Unit weight
- Grading
- Porosity
- Void ratio
- Permeability
- Compressibility
- Shear strength
- Atterberg Limits (Liquid limit, Plastic limit, Shrinkage limit)
D. These physical properties are affected by four main factors:
- Predominant size of the mineral particles
- The type of mineral particles
- The grain size distribution
- The relative quantities of mineral, water and air in the soil matrix
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(Posted by Eric E. Jorden, M.Sc., P.Eng. Consulting Professional Engineer, Forensic Engineer, Geotechnology Ltd., Halifax, Nova Scotia, Canada December 21, 2021. ejorden@eastlink.ca)